It is known that grain is degraded as a result of falling at high velocity. The damage occurs both during fall in a downwardly inclined conduit, as well as upon impact after discharge from the conduit. The damage is apparently caused by particle-to-conduit abrasion and particle-to-particle impact, and causes fines and dust.
It is also known that there exists a particular fall velocity above which the affect on the quality and value of the grain becomes unacceptable. This "damaging velocity" can vary with the type and density of the grain, its dryness, and the type of conduit, but in many cases it is in the vicinity of about 1600 feet per minute. If in falling through a conduit the velocity of the grain exceeds the threshold of damage rate for that stream, grain quality is affected to an unacceptable degree.
The velocity increase is of course most rapid in a vertical conduit, but even in a slanting conduit, velocities of several hundred feet per minute are attained in a few feet. A free-falling stream tends to reach a terminal velocity because the air currents cause turbulence within an unconfined stream; but streams in conduits can reach velocities well above terminal velocities upon discharge. In a long conduit, velocity of the stream can exceed even two thousand feet per minute. The damaging value is apparently not highly dependent on the angulation of the conduit; that is, the damaging velocity is roughly the same for a given stream whether it is falling in a vertical conduit or an angulated conduit.
For a given type of grain, a velocity of 1600 feet per minute, which as noted is the approximate damaging velocity in many cases, may be attained in a vertical spout in roughly a 12 foot drop; in a spout angulated at 30.degree. to vertical, that damaging velocity is attained in a run about 18 feet long (corresponding to a 15 foot drop); and in a spout which is angulated at 45.degree. that damaging velocity is attained in a spout about 33 feet long. It will be appreciated that conduits of those lengths or more are common in most grain handling facilities. The same phenomenon of unreasonable damage to the material as the result of high conduit velocities also arises with other materials than grain, although it is most important in respect to grain because of the relative frangibility and layered structure of grain kernels.
To prevent a falling particulate stream from exceeding damaging velocity, the most straightforward approach is to limit the distance of the drop or to angulate the conduit so that the velocity increase is less. However, it is difficult to avoid the need for a substantial elevation change in many instances, as for example in loading grain into the hold of a ship.
It is also possible to impose a flow restrictor in the path of the falling grain to slow its passage. So called "dead boxes" are known for that purpose, which comprise a narrow fixed throat section which can be mounted directly in the conduit. However, dead boxes are really effective only in those situations where the grain flow rate (i.e., bushels per hour) is constant within a relatively small range and where there are no "surges." That is not usually the case. In the general situation, surges--abrupt increases in flow rate, followed by decreases--occur repeatedly. Surging occurs, for example, when a conveyor bucket empties into the upper end of the spout; a period of lower flow may follow, until another bucket refills the chute. Non-uniform moisture content is another cause of surges.
I have observed that where the flow is uneven, use of a fixed restrictor to retard flow is of little effect. In periods of low flow rate, the small stream passes almost unrestrained through the throat opening; and at periods of heavy flow, a "head" of grain builds up rapidly above the throat. This can cause bridging or clogging which can choke off flow completely.